The value of energy storage systems to electric power utilities has increased as the cost of basic energy sources such as oil and gas has continued to escalate. These systems store energy during periods of excess power output in off-peak periods when the demand for electricity is low. They then supply electricity during peak load periods to augment normal turbine-generator power production.
To data SMES has been the only known method for the direct storage of energy as electricity, and as such is the most efficient method. The energy is stored in the magnetic field of a superconducting inductor carrying a dc supercurrent. Other devices convert the excess generated electricity into other forms of energy such as hydro-mechanical, compressed air, thermal, flywheel, chemical (batteries), etc. Such devices must then must reconvert the energy back for use as electricity, thus making such devices relatively inefficient.
Because of its rapid response time, SMES can also enhance electric power stability by damping unwanted power oscillations and by providing voltage regulation. Power oscillations can be caused by various factors such as a large separation of major load and generation centers, inductance-capacitance coupling in the power circuit, and turbine-generator shaft oscillations.
The round-trip ac-dc-ac efficiency of about 90% for SMES is limited primarily by refrigeration which is in turn related to cryogenic losses, such as heat leak and power losses in the superconductor. The dc current circulates with no power loss in the superconducting inductor. There is a power loss in the superconductor during conversion from ac to dc, and from dc to ac. Despite the high efficiency of SMES, it has a major drawback of high capital cost per kWh of stored energy. This requires a very large system to be competitive with other storage systems. Both the capital cost per unit energy stored, as well as the overall capital cost is high compared with other storage systems. SMES capital cost is approximately proportional to system size raised to the two-thirds power, giving an economy of scale. (This is like a surface-to-volume ratio in which the materials and related costs scale like a surface and the stored energy is proportional to a volume.) Thus if the system size is doubled, the cost per unit energy stored will be about 80% of the original cost. There is a limit to increasing the size beyond the ability of a utility to bear a huge financial burden which is over a billion dollars, as well as site limitation problems for a huge SMES facility.
A number of things contribute to the high capital cost of SMES. One is the high cost of making superconducting wire or cable and forming it into a coil. Over half of the costs of SMES are related to the conductor coil material (low temperature superconductor plus the stabilizing normal conductor, Al), its axial support structure, and fabrication cost. Presently only the low temperature metallic superconductors are applicable to SMES, as the high temperature oxide superconductors are greatly limited in both their current carrying capacity (low critical current density) and in their brittleness. In either case (low or high temperature superconductor), eliminating the need for superconducting wire or cable as in the present invention will reduce capital cost.
At present SMES is a very low temperature system limited to operation at liquid helium temperature (4.2K) and preferably superfluid helium temperature at 1.8K to effect a reduction in overall costs. An expensive closed cycle refrigerator maintains the most expensive cryogen, helium, at 1.8K. In order to reduce heat leak to the coil, the low temperature components operate inside a vacuum insulated cryogenic enclosure (dewar) that surrounds the helium vessel. This vessel must be completely tight making it quite expensive, as a single pinhole would cause disastrous loss of the superfluid helium. Superfluid helium not only has the largest known heat transfer capability for cooling the superconducting coil, but it also has no viscosity and would quickly drain out through a pinhole. An expensive vacuum pump-down system is used to evacuate the dewar which must be leakfree to air. Thermal radiation shields are present in the dewar to reduce heat leak from the 300K (ambient temperature) support structure (bedrock or just earth). A support structure is necessary as both the stored energy density and the pressure produced by the magnetic field of flux density B are proportional to B.sup.2. For example for B=5 Tesla (50,000 Gauss), the stored energy density would be 10,000,000 Joules/m.sup.3, and the pressure would be 100 atmospheres. For B=10 Tesla, the stored energy density would be 40,000,000 Joules/m.sup.3, and the pressure would be 400 atmospheres.